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# When to use composite primary keys

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### When to use composite primary keys

1. 1. By:Alyssa Marie T. Paral Mary May C. Porto
2. 2. 2 Cases of the Composite Primary Keys As identifiers of composite entities, where each primary key combination is allowed once in the M:N relationship.As identifiers of weak entities, where the weak entity has a strong identifying relationship with the parent entity.
3. 3. To illustrate the first case, assume that you have a STUDENT entity set and a CLASS entity set. In addition, assume that those two sets are related in an M:N relationship via an ENROLL entity set in which each student/class combination may appear only once in the composite entity. Shows the ERD to represent such a relationship.
4. 4. The composite primary key automatically provides the benefit of ensuring that there cannot be duplicate values- that is, it ensures that the same student cannot enrol more than once in the same class.
5. 5. A real-world object that is existence-dependent on another real-world object.Those types of objects are distinguishable in the real world.A dependent and an employee are two separate people who exist independently of each other.However, such objects can exist in the model only when they relate to each other in a strong identifying relationship.
6. 6. A real-world object that is represented in the data model as two separate entities in a strong identifying relationship.In both situations, having a strong identifying relationship ensures that the dependent entity can exist only when it is related to the parent entity.
7. 7. In summary, the selection of a composite primary key for composite and weak entity types provides benefits that enhance the integrity and consistency of the model.
8. 8. Figure:5.6 The M:N relationshipbetween STUDENT and CLASS 